It Does What?!

Like the platypus and the lungfish, horseshoe crabs are what biologists refer to as “living fossils,” meaning their basic form has gone essentially unchanged for many millions of years. In the case of horseshoe crabs, fossils as old as 445 million years have been found that are quite similar to the extant species of today.

Despite their common name, the Limulidae aren’t true crabs. They’re arthropods, like crabs, but are actually more closely related to spiders and scorpions. In fact, beneath that tough shell, they do look quite spider-like. If spiders had tails, that is.

Horseshoe crabs live in shallow coastal waters, feeding off worms and molluscs from the ocean floor. They are able to feed in near complete darkness at night due to a remarkable visual system. The creatures have three different types of eyes – compound, median, and rudimentary – located to both sides and to the front of their shell. What’s more, their compound eyes become a million times more sensitive to light at night than they are during the day. Since that’s roughly how much less light they have to work with at night, the crabs are able to see equally well at night and during the day.

Most people who have observed horseshoe crabs know them from their unusual breeding habits. Each spring and early summer, male crabs will search out a mate and attach themselves to the female’s shell using a special modified leg. Then, during the highest tides of the year, usually at night, the females crawl up onto shore by the hundreds, carrying their male cargo. Having picked a spot that’s moist, but not so low as to be washed away with the tide, they dig a nest into the sand and lay their eggs. The attached males get first dibs at fertilising the pre-laid eggs, but must share the task with numerous mate-less onlookers who rush in to get their shot at fatherhood as well (crabs are so uncouth). Since eggs number in the tens of thousands per female, many will probably be successful. Most of these thousands of eggs, however, will become food for migratory birds, who appreciate the extra protein snack on their long journeys. After a month or so, the uneaten eggs will hatch into larvae, which remain on the beach in groups for a couple of weeks before moulting into juvenile horseshoe crabs and finally moving into the water.

Now you might be thinking, “That’s all well and good, but what can horseshoe crabs do for me?” Well, as it turns out, these creatures are some of the most prolific blood donors on Earth (whether they like it or not). Like our friend Mr. Spock, horseshoe crabs have copper-based blood, rather than the iron-based concoction favoured by humans. They are literally blue-blooded. And instead of white blood cells to fight off infection, they have amebocytes. These amebocytes are so valuable in detecting certain types of bacterial infections in humans that a quart of horseshoe crab blood is worth approximately $15,000 US. Crabs are caught, transported to a lab, and drained of about 30% of their blood before being released. The company behind this 50 million dollar per year industry states that only about 3% of the quarter million crabs die from the procedure annually, while other studies have found the number to be nearer to 15% (read more about it here). Knowing who’s right may become very important, as horseshoe crab populations are declining worldwide, additionally affecting the migratory birds that feed on their eggs. Either way, next time you survive an E. coli infection, thank a horseshoe crab.

About This Blog

It has been said that every living species represents a unique set of solutions to a unique set of environmental challenges, and we have the endless trial-and-error of natural selection to thank for those solutions. The adaptations that enable organisms to survive and flourish are often elegant in their simplicity- testaments to the beauty of nature. At other times, the solutions are, well... questionable. This blog is dedicated to exploring all the wonderful freaks of nature and the strange twists and turns evolution has taken along the way.

Click here to subscribe to my monthly science newsletter, A Feast for the Curious.

You’ll receive a single monthly newsletter (and nothing else!) with my most recent posts & publications, as well as a selection of the most interesting articles from around the web on evolutionary biology, botany, zoology, science history, & scientific illustration.

Life is a feast for the curious… keep your appetite up.

Follow Us via Email!

Enter your email address to follow this blog and receive notifications of new posts by email.